Currently, wind energy has been consolidated as the firmest alternative to conventional energy sources. This consolidation is due to an improvement in the technology used, which has made it possible to enjoy an enormous growth in the number of wind generators and wind farms installed. However, this growth could be slowed down by integration problems in the power grid, due to the high degree of wind energy penetration therein.
One of the main problems of wind generators is linked to its performance towards events produced in the power grid, such as voltage dips. It is for this reason that nowadays most countries have been forced to regulate the performance of wind generators against said voltage dips, and these regulations are becoming stricter and stricter.
A large number of the wind turbines installed use Doubly Fed Induction Generators (DFIG) and full converters (FC). Both technologies are based on an electric generator connected to the grid through a back-to-back or AC/DC/AC (alterating current-direct current-alterating current) converter. In the case of DFIG systems, the converter is connected between the generator rotor and the power grid, while in the case of FC systems it is connected between the generator stator and the power grid.
Said AC/DC/AC converter basically comprises a generator-side converter and a grid-side converter, both connected by means of a DC bus.
One of the main drawbacks of said generators is their performance when faced with grid events, particularly voltage dips. In the case of DFIG systems, said voltage dips cause the appearance of elevated transient currents in the generator-side converter, which can cause serious damage to said converter, even causing the destruction thereof. In the case of FC systems, the appearance of voltage dips limits the discharge of power to the grid.
One of the most used conventional solutions to resolve this drawback is to include a chopper in the DC bus. Operating a load in a controlled manner to discharge power from a continuous bus of a power converter has been done for decades, for example in speed variators. FIG. 1 represents this solution, which forms part of the state of the art (Source: Power electronics: converters, applications, and design, Ned Mohan, Ed. John Wiley & Sons, 1989, Page 421, FIGS. 14-20 (a)).
One example of an operating method of a chopper is disclosed in JP7194196. Said chopper includes several resistive branches which are activated and deactivated according to the voltage level in the DC bus.
Other examples of operating a chopper are found in American patent U.S. Pat. No. 7,015,595, which discloses a chopper method and an operating system. Likewise, patent application US2009079193A1 discloses a control method for a chopper with two branches.
Most of the controls associated to a chopper implement a hysteresis control. Hysteresis control is a simple control which activates or deactivates the chopper according to two pre-set voltage levels of the DC bus of the power converter. In the event that the converter is used in a connection application to the grid in which a quick response is required against grid transients, this control does not permit sufficiently fast dynamics, so that the bus voltage can exit the range of normal operating conditions.
Another solution proposed by the state of the art is to control the chopper by means of a PWM modulation (Pulse-Width Modulation). This modulation maintains the voltage level of the bus at the desired level, but requires a more complicated and expensive hardware control than the solution proposed by this invention.